Domestication can serve as a study system of rapid evolutionary change with wide-ranging effects on traits in animals. The chicken was domesticated from the Red Junglefowl and has diverged in behaviour, morphology and life history traits. Conversely, feralisation is a more recent process when domestic animals are again exposed and respond to an environment outside of human husbandry. Linkage-based quantitative trait locus (QTL) mapping has been used to localise genetic variants that affect domestication traits in the chicken genome. Because of the limited resolution of linkage mapping, the QTL regions associated with domestication traits are often broad and contain many genes. One approach to help sort out potential causative genes is to measure gene expression as an intermediary molecular phenotype. In this dissertation, expression quantitative trait locus (eQTL) mapping of gene expression traits is used to search for potential causative genes for domestication traits in the chicken. Expression quantitative trait loci were mapped across the whole genome in bone and hypothalamus samples, and targeted at QTL regions in the base of the comb. These studies have resulted in candidate quantitative trait genes, supported by genetic and gene expression evidence, for relative comb mass, bone allocation, egg production and fearful behaviour as measured in an open field test. Secondly, a population genomics approach was used to study the molecular basis of feralisation in a free-range feral chicken population from the Pacific island of Kauai. Mitochondrial DNA sequences and phenotypic observations establish the hybrid origin of this population as a mixture of wild and domestic chickens. Genome-wide mapping of pooled heterozygosity highlight regions that may be involved in adaptation to the feral environment. The expression QTL results bring us closer to knowledge about the molecular basis of domestication traits in the chicken, suggesting plausible candidate genes and opening up for functional studies of individual loci. The population genomic study shows that feralisation has a mostly different genomic architecture than domestication, and suggests phenotypic effects, based on overlap with domestication QTL regions, for some of the identified regions.